Information storage apparatuses (optical disk apparatuses) are practically used that write information on an optical disk, or read information written on an optical disk using a laser beam output from an optical pickup.
As the performance of personal computers (PC) is being improved, the personal computers become more capable of handling even audio-visual information such as music and video. Audio-visual information is usually very large in memory size. Optical disks have drawn attention as recording media for storing audio-visual information. Because the cost of optical disks is being reduced, as well as their high memory capacity, optical disk apparatuses have became the popular peripherals of personal computers.
Typically, a rewritable recording medium such as a CD-R/RW disk (Compact Disk-Recordable/Rewritable), a DVD−R/RW disk (Digital Versatile Disk), and a DVD+R/RW disk, is formatted so that, when rotated in a constant linear velocity mode, wobble signal detected from wobbling tracks formed on the recording medium becomes constant in order to accurately detect the linear velocity at each radial position.
Wobble signal always needs to be detected, regardless of reading or writing. However, data written on the recording medium and/or modulated laser beam for writing data become the source of noise, and may degrade the quality of wobble signal. That is, wobble signal is included in reflective light from a track, but the reflective light complicatedly includes components that becomes noise to the wobble signal due to data written on the optical disk and/or the power modulation of laser beam for writing data on the optical disk.
Wobble signal is generally extracted in the following manner. The reflective light from a track is received by a photo detector divided into two parts with respect to tangential direction to the track. Noise component included in the reflective light is removed by subtracting output signal from one part of the divided photo detector from output signal from the other part.
Before an optical disk apparatus is shipped from a manufacturing plant, the divided photo detector is aligned so that the reflective light from a track hits the center of the photo detecting surface of the divided photo detector. However, the reflective light may deviate from the center of the photo detecting surface due to temperature change during operation and/or change over time due to vibration. In such a case, because the output signal from each part of the divided photo detector includes different noise component, the noise components may remain un-canceled even if output signal of one part is subtracted from output signal of the other.
To solve such a problem, Japanese Laid-Open Patent Application No. 8-194969, for example, discloses a method of detecting wobble signal in which data signal component can be clearly removed by normalizing (so-called amplitude regulation automatic gain control (AGC)) the amplitude of output signals from a photo detector divided in the tangential direction of a track and by generating the wobble signal based on the difference between the output signals. This application further discloses an embodiment in which, when an optical disk on which data has not yet been written is read, amplitude regulation AGC is not used since the data signal component does not need to be removed. A determination is made of whether the amplitude regulation AGC circuit is used based on a determination of whether data has been written on the optical disk.
This application No. 8-194969 provides a method of detecting wobble signal in which wobble signal of high quality can be obtained during reproduction of signal from an optical disk. However, the method disclosed in this application is not applicable to wobble signal detection while data is being written.
Particularly, the above method does not take into account the switching of operation from data-reading to data-writing, and fails to obtain good wobble signal during time period in which reproduction is ended and recording is started. As a result, accurate write clock cannot be generated based on such a wobble signal of low quality. That is to say, laser power for data-reading is different from laser power for data-writing, and as a result, the levels of detected wobble components are different. As will be appreciated, when operation is switched from data-reading to data-writing, if the operation is not adequately controlled, the slice voltage of made-binary wobble signal may be deviated (hysteresis, for example). The deviation may cause jitter (deviation over time), which may result in that PLL circuit fails to lock in write clock and operates unstably.